Sella Turcica Shape in Fragile X Syndrome

Discussion

The diagnostic significance of the sella turcica is primarily the bone’s relationship to its essential functional content, the pituitary gland. Pathologies of the sella turcica are usually not early signs of disease of the pituitary gland (6). The interpretation of radiologic changes of this bone segment on plain radiographs for neurological diseases have become largely diagnostically irrelevant due to the superior sectional imaging techniques of magnetic resonance tomography and computed tomography in this field (6). Also, in standardized lateral cephalometry, even significant differences in the size and contour of the sella are often considered normal variations (6). However, the morphology of the sella turcica may offer clues to syndromic disease which can be assessed on plain radiographs (7, 8). Standardized cephalometric posterior-anterior or lateral projections are usually obtained for radiological survey of the skull in suspected syndromic skull malformations to make an initial assessment of the craniofacial skeleton (6, 7). On lateral cephalograms, sella turcica is a characteristic cup-shaped indentation in the center of the skull base facing the brain. On lateral cephalograms and CBCT, the shape, size, inclination, and thickness of the walls as well as relationships to neighbouring skull base regions are assessed (7, 8). However, the shape and size of the sella are very variable (9-11). The type of sella in the sagittal section corresponds most closely to the oblique anterior sella according to Axelsson et al. (12).

Fragile X syndrome (FXS) and sella turcica. There is some information on the morphology of sella turcica at FXS. Kjaer et al. (3) examined 7 cases with FXS radiologically regarding skeletal dysplasia (age 2 years, 9 months to 21 years, 2 months, males: 6, female: 1). The sella was malformed in all cases where skull x-rays were available for study (n=5). The findings were individually variable. Oblique anterior wall of the sella was prominent in 2 cases, giving the impression of a cranially ‘open’ sella turcica (3). The dorsum sellae was extremely short developed in other two cases. One case had developed a sella bridge. The reproduction of radiological details in the figures of the patients represent only findings of males (3). In comparison to the reported five cases, the present case also shows an oblique, shallowly developed anterior sella wall merging into the sella bottom (Figure 1 and Figure 2). In contrast to previous reports (3, 4), the dorsum sellae is long, thin, and slightly curved. An ‘open’ sella (3) cannot be deduced from the radiographic findings. A steep inclination of the clivus was registered in 2 cases (3). This finding is also evident in the presented case. The volume of the sella is within the range of normal sella volume values published in recent studies (9-11).

Further studies on embryological specimen with known FXS revealed developmental defects of sella turcica (4). The authors examined the sella turcica in two embryos and recorded a pronounced notching of the anterior wall in one case and a very short dorsum sellae in the second case (4). Both findings were absent in the present case. The development of the dorsum sellae is controlled by the notochord (7, 8). Nasion-Sella-Basion (NSBa) angle (inclination between anterior and posterior skull base) of the patient is below the expected mean value (131.22°±4.73) (13).

Triple X syndrome (TXS). In principle, the patient’s known TXS could have had an influence on the skull base development. However, the facial findings of affected females are not very characteristic. A conspicuous radiological skull base has not been described so far in TXS. Reference bias to TXS may have an impact on the estimation of the frequency of syndrome-associated malformations (14). To our knowledge, cephalometric studies on TXS are not available. Isolated cases with facial co-morbidities potentially influencing bone development are probably sporadic (15). A case report with TXS and aplasia of a central incisor of the maxilla is interesting (16), because the fronto-maxillary developmental field is affected, whose central cranial endpoint is localized in the anterior wall of the sella (7, 8). Overall, an influence of the TXS on the sella morphology cannot be substantiated according to the current state of knowledge.